Prior art phototypesetting machines employ slidable carriages which carry optical elements such as lenses for imaging characters upon a photosensitive medium. Required changes in magnification in turn require the shifting of the carriage positions along the optical axis (in the z direction) to maintain focus for all character image sizes imaged upon the photosensitive medium. It has been desirable in the design of certain of these phototypesetters to provide for shifting of the lens carriages along left-hand and right-hand rail elements or ways in a manner to maintain translation of the carriage mounted lens elements in space (in the x and y directions) at a minimum as the carriages are moved parallel to the longitudinal axis of the ways supporting the carriages. This requirement tends to conflict with the requirement that these individually actuated carriages do not bind since such binding would render the system subject to partial or total jamming. In order to accommodate these somewhat conflicting requirements, the ways supporting the carriages had to be precisely machined and precisely positioned with respect to each other, which is relatively costly. Even with such precise fabrication, a certain amount of looseness or play had to be built into the system to prevent binding of the carriages with respect to the supporting ways, the play subjecting the lens elements to random uncontrolled displacement in x and y.
It is accordingly highly desirable to provide a lens element carriage arrangement which eliminates the abovestated looseness or play and additionally, does not require precise positioning or parallelism between the two ways supporting the carriages. The elimination of the latter requirement considerably reduces manufacturing costs, by for example, permitting the ways to be affixed to a bed which need not maintain precise dimensioning and thus may be extruded rather than otherwise precision machined.